The present invention relates generally to the recovery of tungsten, and in particular to its recovery from tungsten concentrates. More particularly, the present invention relates to conditioning tungsten concentrates prior to leaching the concentrates for production of an aqueous tungstate solution.
Tungsten (as WO.sub.3) is customarily concentrated from associated material by any of several techniques, including froth flotation and gravity concentration. These techniques are effective to concentrate tungsten from feed material such as scheelite (CaWO.sub.4) or wolframite ((Fe, Mn)WO.sub.4) ore, or tungsten-bearing byproduct material formed in the recovery of other metals with which the tungsten is found. In froth flotation, the feed material is customarily crushed and slurried in water, one or more flotation agents such as conditioners, collectors, or frothers are added to the slurry, and the slurry is subjected to froth flotation to recover the scheelite as a flotation concentrate and to reject gangue. In gravity concentration, a slurry of crushed feed material is stratified in a moving stream of water, causing the various solid components of the slurry to collect in fractions roughly corresponding to the relative density thereof. Gravity concentrates are typically recovered in spiral classifiers and shaker tables.
The tungsten concentrate, however recovered, can then be leached in an aqueous alkaline solution of an alkali metal carbonate or hydroxide, to solubilize the tungsten values in an alkali metal tungstate leach liquor. The aqueous leach liquor can then be treated by various steps to further purify the tungsten, typically leading to a purified ammonium tungstate solution from which a solid ammonium tungstate produce can be recovered.
Tungsten flotation concentrates and gravity concentrates typically contain impurities which physically or chemically interfere with the leaching process and with subsequent purification steps. The impurities or their reaction products can enter into the leaching reaction, in which case they also undesirably consume leaching reagent. In particular, tungsten flotation and gravity concentrates tend to contain amounts up to several percent or more of sulfides, particularly as pyrrhotite (FeS) as well as pyrite (FeS.sub.2) and chalcopyrite (CuFeS.sub.2). Sulfides react with the alkaline leaching reagent, and form a variety of contaminant compounds which interfere with subsequent purification steps and which can reduce the purity of the product. It is therefore highly desirable to be able to treat tungsten concentrates prior to leaching thereof to remove sulfides from the concentrates.
Roasting sulfide-bearing concentrates to oxidize the sulfides partially or completely is a generally known expedient in a variety of contexts. Typically, sulfides are converted to gaseous sulfur dioxide, which is evolved and collected in a scrubber or other suitable emission control device. However, simply roasting tungsten concentrates to oxidize completely the sulfides contained therein can lead to reduced leaching efficiency as well. Tungsten concentrates typically contain calcium, present for instance as calcite (CaCO.sub.3), in sufficient amounts to form substantial quantities of calcium sulfate in the roasted product by reaction with sulfur dioxide formed during roasting. Calcium sulfate reacts with the sodium carbonate agent used to leach the roasted concentrate, thereby increasing the amount of leaching reagent required for a given quantity of tungstate fed, and greatly reduces leaching efficiency. Calcium sulfate also adds to the overall solids load that must be handled in the tungsten recovery process.
Roasting calcite-containing scheelite concentrates presents the additional drawback that free lime, CaO, can be formed in the roasted product. When sodium carbonate is used as the leaching reagent, free lime present in the roasted concentrate can react with the sodium carbonate to form calcium carbonate and undesirable amounts of sodium hydroxide. The sodium hydroxide in turn reacts with silica and other impurities, thereby forming soluble compounds which contaminate the leach liquor and insoluble compounds which promote the formation of scale in the process equipment.
Accordingly, there is a need for a process for conditioning tungsten concentrates to oxidize sulfides therein which also permits efficient leaching of tungsten from the conditioned concentrate unimpeded by calcium-bearing byproducts. Such a conditioning process should be amenable to treatment of tungsten flotation and gravity concentrates having a range of sulfide and calcite contents.